[Python-3000] Implementations: A syntax for extending method dispatching beyond type/class inheritance (original) (raw)

Bill Janssen janssen at parc.com
Thu Dec 7 17:25:26 CET 2006

Dave, thanks.

To me, even if Car happens to represent a dict by being able to be used as a dict, the expression of this fact feels better expressed as merely implementing dict.

compare: class Car(dict): # car... is a dict? hmm, a bad ring to it ... class Car: implements dict # oh, Cars can be used like a dict, got it ...

OK, I guess the difference is that, for me, (1) this doesn't happen a lot (ever?), and (2) I don't feel that the nuance here is important enough to add another mechanism to the language which needs to be supported. I'd rather just use type inheritance.

Without something like an implements declaration, if I ever want to express a relationship to another class, but with different internals, I am forced to first define an abstract class and then inherit from that. With an implements declaration, I can say implements dict and be free of inheriting any of its internal implementation.

That's why it's important for Python to have a clear set of base interface types for people to use.

In the syntax sketch, an Implementation is basically a list of types/class, methods, and other Implementation lists.

That sounds a whole lot like a Python 2.5 class, to me.

Bill

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